African Journals Online
WaterSA

Volume 27, Issue 2 2001
ABSTRACTS

The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal: Part 5: Experimental periods using a ferrous-ferric chloride blend

De Haas, D.W.; Wentzel, M.C.; Ekama, G.A.

Abstract: A blend of ferrous chloride and ferric chloride (FeCl₂-FeCl₃) was simultaneously dosed into an activated sludge system at pilot scale in order to test the effect on biological P removal. Additional removal due to chemical precipitation was measured as the difference in system P removal between parallel test and control systems. Both systems strongly exhibited biological excess P removal (BEPR). The extent of P release in the anaerobic reactors of the two systems was compared by mass balance, as one indicator of the relative "magnitude" of BEPR. Phosphorus fractionation of the mixed liquor also served as an indicator of the biological and chemical mechanisms. Evidence was found that the BEPR mechanism is partially inhibited by simultaneous FeCl₂-FeCl₃ addition, even in the absence of effluent phosphate limitation. However, the degree of inhibition was relatively low, ranging from 3 to 25% (approximately) for Fe doses in the range ca. 10 to 20 mg/ℓ as Fe, with an average system P removal of 14 to 18 mgP/ℓ in the control. FeCl₂-FeCl₃ dosing in this range was sufficient to produce additional P removal of the order of 1 to 8 mgP/ℓ over periods of one to seven sludge ages per experimental period, depending on the experimental conditions. Sustained operation of the BEPR mechanism in the presence of FeCl₂-FeCl₃ was possible over a continuous period of seven sludge ages, under conditions in which effluent phosphate was at least partially limiting. Under such conditions, the chemical and biological mechanisms appear to be "disadvantaged" to approximately the same extent, as evidenced by the apparent stoichiometry of Fe: P for the chemical precipitation and magnitude of the poly P containing fractions measured for the biological mechanism. This suggested that the biological mechanism is able to compete effectively with the chemical mechanism under conditions of low reactor phosphate concentrations (~1 mgP/ℓ orthoP) for sustained periods. However, the presence of simultaneous chemical precipitant significantly reduces the extent to which the biological P removal potential is utilised under P-limiting conditions. This could explain the difficulty sometimes reported in the control of full-scale activated sludge systems with simultaneous precipitant addition where a very low effluent P concentration

The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal: Part 6: Modelling of simultaneous chemical-biological P removal - review of existing models

De Haas, D.W.; Wentzel, M.C.; Ekama, G.A.

Abstract: This paper reviews three published models for simultaneous chemical phosphorus precipitation in activated sludge systems using metal salts. In the first, a chemical equilibrium approach is used, based on observations made from batch and continuous-flow tests, a theoretical formula for metal (e.g. ferric) hydroxy-phosphate and a set of metal phosphate complexes or ion pairs for dissolved orthophosphate (orthoP) species. Apart from applying the precipitation stoichiometry observed in admixture with activated sludge, in this model no interaction between the chemical and biological mechanisms is accounted for and no biological processes are modelled. In the second model, a combined equilibrium-kinetic approach is used to model the chemical and biological processes. The chemical and biological processes become kinetically linked through soluble orthoP as a variable. This model includes biological processes for conventional activated sludge systems, but does not include biological excess P removal processes (BEPR). Apart from this limitation, a potential problem in the combined equilibrium-kinetic approach was identified: The precipitation reactions were modelled based on equilibrium chemistry and assumed to be complete at the start of simulation; precipitate, therefore, could not form dynamically during the ensuing kinetic simulation. Furthermore, the model predictions were very sensitive to the choice of certain key equilibrium (or solubility product) constants. The third approach was to model the precipitation (and dissolution) reactions as kinetic processes within a fully kinetic model for activated systems, including the processes for BEPR. This approach depends on the appropriate selection of rate constants for the forward (precipitation) and reverse (dissolution) reactions. In effect, a number of reactions from equilibrium chemistry are combined and replaced with one "surrogate" reaction having its own apparent equilibrium constant. The kinetic approach offers a number of advantages but is still subject to the limitation that it requires calibration against actual data from activated sludge systems in which simultaneous precipitation is applied. Moreover, interaction between the chemical and biological P removal mechanisms in the model is confined to "competition" for available soluble orthoP. This aspect requires further examination.

The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal: Part 7: Application of the IAWQ model

De Haas, D.W.; Wentzel, M.C.; Ekama, G.A.

Abstract: The IAWQ Activated Sludge Model (ASM) No. 2 is a kinetic-based model and incorporates two simple processes for chemical precipitation and redissolution that are readily integrated with biological processes for carbon, nitrogen and phosphorus removal. This model was applied to experimental data collected as part of this study from parallel pilot-scale 3-stage Phoredox systems with and without simultaneous dosing of chemical precipitant. The precipitants tested were alum, ferric chloride and ferrous-ferric chloride. The model was calibrated to the control unit (without precipitant addition) in order to match effluent phosphate (P) predictions (and hence P removal) as closely as possible. The same calibration was then applied to modelling the test unit (with precipitant addition). It was found that the default model input stoichiometry for the precipitation reaction (ideal 1: 1 molar ratio of metal ion (Me) to P) was suitable for ferric chloride addition at a 20 d sludge age, but did not accurately reflect the test system behaviour for all experimental periods. A lower stoichiometry (0.60 to 0.75) was required for alum at a 20 d sludge age, and for a blend of predominantly ferrous chloride at a 10 d sludge age. The input stoichiometry was further decreased under P-limiting conditions. A simple approach to, and possible reasons for, the manipulation of the model stoichiometry are discussed in the light of observed stoichiometry from system P removal and fractionation data collected as part of this study. Furthermore, an alternative approach based on manipulation of the precipitation (and hence redissolution) kinetic constant is suggested and evaluated using available experimental data. Model predictions and observed data in respect of polyphosphate (polyP) and suspended solids are also compared and discussed. It is concluded that the ASM No. 2 model provides a useful basis for modelling simultaneous P precipitation, provided certain minor modifications are made. Further investigation into the kinetics of the precipitation process(es) is recommended, particularly in relation to the effect of system sludge age. The model lends itself to further enhancement by incorporating additional physico-chemical processes.

The application of different techniques to determine activated sludge kinetic parameters in a food industry wastewater

Contreras, Edgardo; Bertola, Nora; Zaritzky, Noemi

Abstract: In the present work, a continuous-feed system under steady state conditions (classical method) and a respirometric technique based on oxygen consumption measurements, were used to compare microbial parameters using a wastewater model system of a potato processing plant. The effects of the operating conditions in the continuous aerobic reactor on microbial growth and flora composition were also analysed.

Continuous-feed experiments allowed for the calculation of the following biodegradation parameters: maximum substrate consumption specific rate (q_Smax) = 0.19 mgCOD (mgVSS)^-1·h^-1, saturation constant (Ks) = 8.3 mgCOD·l^-1, biomass yield (Y_X/S) = 0.61 mgVSS (mgCOD)^-1, biomass decay constant (kd) = 0.04·h^-1 and maximum oxygen consumption specific rate (q_O2max) = 0.03 mgCOD (mgVSS)^-1·h^-1.

The respirometric technique also allowed for the calculation of kinetic parameters. However, these parameters depended on dilution rate (D) and / or on dissolved oxygen concentration (DO). Values of q_O2max and Ks increased with D; q_O2max ranged between 0.05 and 0.13 mgO₂ (mgVSS)^-1·h^-1 and Ks between 2 and 26 mgCOD·ℓ^-1 for D values varying between 0.01 and 0.15·h^-1.

Microscope observations showed that sludge composition was a function of dilution rate. Low D values (low soluble organic matter (Ss) and high DO concentrations) yielded sludges mainly formed by floc-forming bacteria; high D values (high Ss and low DO concentrations) yielded sludges mainly formed by filamentous micro-organisms.

Since the low substrate / biomass ratios used in the respirometric method limit the growth of micro-organisms maintaining the initial physiological state of the original biomass, the observed differences in the respirometric parameters reflected the actual microbial composition.

Effect of PAC addition in combined treatment of landfill leachate and domestic wastewater in semi-continuously fed batch and continuous-flow reactors

Cecen, Ferhan; Aktas, Ozgur

Abstract: The combined biological treatability of landfill leachate and domestic wastewater was investigated in both semi-continuously fed batch (SCFB) and continuous-flow (CF) activated sludges with recycle. Powdered activated carbon (PAC) was added in order to investigate the improvement in organic carbon removal and nitrification. The results obtained in SCFB and CF operations were compared to each other. In both types of operations, COD and ammonia removal efficiencies decreased with an increase in the leachate ratio of the total wastewater. As the leachate ratio increased, the positive effect of PAC on COD removal and nitrification became more apparent. In SCFB-type operations, nitrification was more inhibited than in CF operations. Additionally, the enhancement of nitrification was more apparent in CF operations than in SCFB operations where there was PAC addition. In CF operations, sufficient PAC addition could completely prevent nitrification inhibition and nitrite accumulation was avoided. With regard to nitrification, the positive impact of PAC was observed after some time since inhibition of nitrifiers was more severe than heterotrophs.

Full-scale ANANOX^(R) system performance

Garuti, Gilberto; Giordano, Andrea; Pirozzi, Francesco

Abstract: This paper reports the results of the first experimental investigations carried out on the only existing full-scale plant that makes use of the biological treatment system known as ANANOX^(R). This system was first set up by the Italian research staff at ENEA (Agency for New Technologies, Energy and Environment) and consists of two biological stages in series: a two-section ABR and an anoxic section followed by an activated sludge process. The investigation aimed primarily to assess system performance under uncontrolled load conditions. In particular, system efficiency was assessed with regard to carbonaceous and nitrogen compounds in the anaerobic and anoxic phases, and the role of sulphides in the denitrification process was examined.

The results obtained show the system's ability to ensure efficiency levels that comply with stringent effluent regulations while also allowing considerable savings in running costs.

Economic study of the treatment of surface water by small ultrafiltration units

Drouiche, M.; Lounici, H.; Belhocine, D.; Grib, H.; Piron, D.; Mameri, N.

Abstract: The purpose of this work is to evaluate the possibility of utilising an ultrafiltration process for the treatment of water from the dam in the Kabylia region of Algeria and, in particular, for the provision of drinking water to people living in dispersed small villages. The water quality was determined by measuring turbidity, and natural organic matter concentration. The results obtained with an ultrafiltration process indicate that this technique can considerably reduce suspended and organic matter. It also improves the bacteriological quality of the treated water. An economic evaluation for ultrafiltration of surface water is presented. The economic study was performed for a drinking water unit of 20 m³/h . It was found that the cost per m³ of treated water ($ 0.235/m³) obtained would not be excessively high for the states of the North African region.

Numerical simulation of baseflow modification due to effects of sediment yield

Mwaka, B.M.L.

Abstract: Alluvial rivers re-shape their own geometry by depositing sediments or eroding the channel when their dynamic equilibrium is disturbed. Such adjustments may induce river-bed and water-level profiles that have significant effects on the interaction of streams and aquifers that are connected hydraulically. Physically-based mathematical modelling affords the opportunity to look at this kind of interaction, which should be simulated by deterministic responses of both water and fluvial processes. In addition to simulating the streamflow and groundwater dynamics, the model should also be capable of tracking down the level of the coupling interface boundary.

A procedure for modelling alluvial stream-aquifer interaction - MASAI - has been developed to enable the coupling of unstable alluvial stream-aquifer interacting systems. Application of MASAI to hypothetical alluvial stream-aquifer systems reveals the complex relationships between individual elements of the systems, and highlights the influence of sediment yield on baseflow.

Estimating continuous monthly baseflow time series and their possible applications in the context of the ecological reserve

Smakhtin, V.U.

Abstract: The paper describes a simple technique for baseflow separation from continuous monthly streamflow records which are widely available in South Africa. The technique employs a digital filter algorithm, which has been previously used only with more detailed daily streamflow records. The example applications of the separation technique in three gauged catchments are illustrated and its possible applications in the context of groundwater and estuarine components of the ecological reserve determination are discussed.

Sustainable development indicators for urban water systems: a case study evaluation of King William's Town, South Africa, and the applied indicators

Morrison, G.; Fatoki, O.S.; Zinn, E.; Jacobsson, D.

Abstract: In the light of the increasing pressures on the world's freshwater resources, changes in the present and future urban water systems are called for in order to achieve sustainable development. The transformation from unsustainable practices demands tools that measure progress and can warn of future trends. Sustainable development indicators (SDIs) have been suggested as tools for a number of fields, including the urban water sector. This case study applied 20 SDIs to an urban water system in King William's Town, a medium-sized city in the semi-arid, mostly underdeveloped Eastern Cape of South Africa. The main aims were twofold: To evaluate the sustainability of the urban water system, and to evaluate the individual factors according to criteria. A final working list of the 15 SDIs was found useful for the study area and was produced for use also in future studies.

An assessment of heavy metal pollution in the East London and Port Elizabeth harbours

Fatoki, O.S.; Mathabatha, S.

Abstract: The distribution of heavy metals (zinc, cadmium, copper, iron, manganese and lead) was investigated in seawater and in sediment samples from the East London and Port Elizabeth harbours. Both are ports of major importance to the area. The aim was to assess the impact of potential pollution sources, mainly from the cities' formal disposal to the sea, from industry and from dockyard and shipping activities around the harbour. At the East London harbour, metal concentrations in sea water range from 0.2 to 72.0 mg·ℓ^-1 for Cd, from 0.6 to 42.6 mg·ℓ^-1 for Cu, from 2.4 to 183.0 mg·ℓ^-1 for Fe, from 0.6 to 16.3 mg·ℓ^-1 for Pb, from 0.9 to 23.9 mg·ℓ^-1 for Mn and from 0.5 to 27.6 mg·ℓ^-1 for Zn. In sediments, metal concentrations using the total digestion method range from 0.12 to 1.63 mg·g^-1 (dry weight) for Cd, 12.7 to 183.0 mg·g^-1 (dry weight) for Cu, 1046.0 to 18 114.0 mg·g^-1 (dry weight) for Fe, 3.2 to 84.2 mg·g^-1 (dry weight) for Pb, 87.4 to5 49.0 mg·g^-1 (dry weight) for Mn, 26.1 to 332.0 mg·g^-1 (dry weight) for Zn. In the Port Elizabeth harbour, the concentration of metals in seawaters varied between 0.3 mg·ℓ^-1 and 4.0 mg·ℓ^-1 for Cd, between 0.5 mg·ℓ^-1 and 11.3 mg·ℓ^-1 for Cu, between 3.7 mg·ℓ^-1 and 21.9 mg·ℓ^-1 for Fe, between 0.6 mg·ℓ^-1 and 4.2 mg·ℓ^-1 for Pb, between 0.7 mg·ℓ^-1 and 16.8 mg·ℓ^-1 for Mn and between 0.7 mg·ℓ^-1 and 16.2 mg·ℓ^-1 for Zn. In sediments, values of metals also using the total digestion method ranged from 0.1 to 1.4 mg·g^-1 (dry weight) for Cd, from 8.6 to 82.3 mg·g^-1 (dry weight) for Cu, from 4219.0 to 15 182.0 mg·g^-1 (dry weight) for Fe, from 9.0 to 61.9 mg·g^-1 (dry weight) for Pb, from 103.0 to 499.0 mg·g^-1 (dry weight) for Mn and from 18.8 to 126 mg·g^-1 (dry weight) for Zn. The results are indicative of the contribution of heavy metal pollution from storm water drains and streams which carry runoff from industrial, urban and residential sources. Ship repair activities are also suspected to be responsible for elevated concentrations in the upper reaches of the harbour.

An assessment of the water quality of the Isinuka springs in the Transkei region of the Eastern Cape, Republic of South Africa

Faniran, J.A.; Ngceba, F.S.; Bhat, R.B.; Oche, C.Y.

Abstract: The physico-chemical properties of Isinuka springs, a "wonder" water resource in Port St Johns area of Eastern Cape Province, were investigated over three seasonal regimes. Water samples were collected from the five spring sources, along Isinuka river and from Ferry Point Cottage spring and analysed for their quality parameters. Most of the water quality variables measured were found to show seasonal fluctuations. Isinuka spring water is not fresh as it contains high salt contents and the results indicate that it is not suitable for drinking without treatment. The water is moderately hard, salty and fairly neutral with pH ranging from 6.87 to 8.33. One of the five sources continually emits a characteristic odour and the five spring sources were characterised by very high concentrations of TDS, turbidity, Cl^-, and NH₄⁺-N which exceeded the maximum permissible levels recommended for drinking waters. The NO₃^--N and NO₂^--N were, however, present at levels far below the critical value of 10 mg/l above which the occurrence of blue baby syndrome (methaemoglobinaemia) due to NO₂^- poisoning might be a problem in pregnant women and bottle-fed infants. Water from Ferry Point Cottage spring met the water quality guidelines for drinking water.

Bacteriophages: update on application as models for viruses in water

Grabow, W.O.K.

Abstract: Phages are valuable models or surrogates for enteric viruses because they share many fundamental properties and features. Among these are structure, composition, morphology, size and site of replication. Even though they use different host cells, coliphages and Bacteroides fragilis phages predominantly replicate in the gastro-intestinal tract of humans and warm-blooded animals where enteric viruses also replicate. A major advantage of phages is that, compared to viruses, they are detectable by simple, inexpensive and rapid techniques. In view of these features, phages are particularly useful as models to assess the behaviour and survival of enteric viruses in the environment, and as surrogates to assess the resistance of human viruses to water treatment and disinfection processes. Since there is no direct correlation between numbers of phages and viruses, phages cannot to a meaningful extent be used to indicate numbers of viruses in polluted water. The presence of phages typically associated with human and animal excreta indicates the potential presence of enteric viruses. However, the absence of these phages from water environments is generally a meaningful indication of the absence of enteric viruses. This is because phages such as somatic coliphages, F-RNA coliphages and B. fragilis phages generally outnumber enteric viruses in water environments, and they are at least as resistant to unfavourable conditions including those in water treatment and disinfection processes. However, using highly sensitive molecular techniques viruses have been detected in drinking water supplies which yielded negative results in conventional tests for phages. Initially, data on phages were rather confusing because a wide variety of techniques was used. However, techniques for the detection of phages are being standardised internationally. This applies in particular to somatic and F-RNA coliphages, and B. fragilis phages, which are most commonly used in water quality assessment. Reliable and practical techniques now available include direct quantitative plaque assays on samples of water up to 100 ml, and qualitative tests on 500 ml or more using highly sensitive enrichment procedures.

A Ceratium hirundinella (O.F. Müller) bloom in Hartbeespoort Dam, South Africa

Van Ginkel, C.E.; Hohls, B.C.; Vermaak, E.

Abstract: During the late winter to early spring of 1999 Ceratium hirundinella was recorded for the first time in the Hartbeespoort Dam, South Africa, and in bloom forming conditions. The C. hirundinella bloom started in July 1999 after complete mixing occurred and a Microcystis aeruginosa bloom disappeared. C. hirundinella occurred in chlorophyll a concentrations up to 4243 mg/l. The clogging of filters at water care works and a fish kill were encountered during the same period, in the area. This paper discusses the extent of the bloom, the possible causes and the water characteristics found in the Hartbeespoort Dam during the development of the bloom.

Fundamental study of a one-step ambient temperature ferrite process for treatment of acid mine drainage waters: rapid communication

Morgan, B.E.; Loewenthal, R.E.; Lahav, O.

Abstract: A novel approach towards the removal of iron and heavy metals from South African acid mine drainage (AMD) waters is presented. The approach involves the controlled oxidation of ferrous-containing AMD water at ambient temperatures in the presence of magnetite seed. The resulting oxidation product is the ferrite (M1³⁺₂M2²⁺O₄) magnetite (Fe₃O₄), which has the capacity for nonferrous metal removal, and which forms a stable sludge that is easily separated from the effluent. Sludge characterisation studies (XRD, SEM and dissolution tests) show that oxidation of ferrous solutions under controlled pH and oxidation conditions (pH 10.5, air flow rate = 0.05 ℓ/min) in the presence of magnetite seed (initial seed: ferrous ratio = 7: 1) yields almost pure magnetite at ambient temperature. It was found that magnetite seed channels the end products of the AMD oxidation reaction towards magnetite. Under identical conditions, but in the absence of magnetite seed, a poorly characterised mixture of largely amorphous iron oxides are formed with magnetite comprising not more than 17% of the total iron. The kinetics of the reaction under the investigated conditions were found to be very favourable, with magnetite forming at a rate of 12.8 mg Fe/ℓ/min. The total iron concentration in the effluent was always less than 1 mg/l representing an iron removal efficiency of 99.9%. The precipitant settled well (SVI 8 mℓ/g) and showed substantial stability at pH 3 (dissolution of 1.1% after 120 h). An outline for a one-step ambient temperature ferrite process is presented.